In MP3 (MPEG-1 Layer 3) encoding and decoding, the spectrum is considered to be divided into 576 frequency bins. These frequency bins are in turn grouped into sub-bands and this grouping of bins into sub-bands is defined in the MP3 specification. The sub-bands themselves are grouped into three regions which are referred to as ‘Big Values’, ‘Count1’ and ‘Rzero’ and the size of these regions may vary according to the particular audio signal. Big Values represents the lowest frequency lines and these are coded with the highest accuracy of all the regions. Count1 represents the higher frequency lines, which are encoded using only a small number of values and Rzero represents the highest frequencies which are removed by the encoder, and hence on decoding these frequency bins are filled with zeros.
The Big Values region is itself divided into three regions (region0, region1, region2) with each of these three regions being encoded using a particular Huffman table. This division of Big Values into three regions typically occurs for each granule when performing MP3 encoding. Some existing encoders, such as LAME (an open source MP3 encoder), use look-up tables to obtain the boundaries between these three regions according to the number of sub-bands to be encoded (e.g. the number of sub-bands within the Big Values region or the number of sub-bands which have any data in them). This requires virtually no computation but does not take into consideration the statistics of a particular audio signal which is to be encoded and therefore can result in encoding redundancies in the signal.